This invention relates to an ion mobility detector in which ions formed in a reactor region of a tubular cell are passed to a drift region of the tube under control of a gate separating the reactor region from the drift region and thence to an electrometer plate.
Ion mobility detectors are known in the prior art. Briefly stated, they comprise tubular structures having a reactor region, a drift region and an electrometer plate. A gas to be analyzed, together with a carrier gas such as air, is introduced into the reactor region where it is ionized, e.g. by a radioactive material contained therein. Voltages applied to a gate between the reactor region and the drift region allow samples of the ions to enter the drift region where they are accelerated by an electric drift field. The ion samples reach terminal velocity dependent on their mass and may be sorted depending on the time they take to travel from the gate to the electrometer plate at the end of the drift region.
The drift field may be produced in various ways, e.g. by ring-like electrodes on a surface (interior or exterior) of an insulating tube to which voltages are applied. Resistors connect adjacent electrodes and a high voltage is applied between the end electrodes. The resistors create an electric field gradient along the tube. Instead of discrete resistors, some prior art detectors use a continuous film of resistive material. See, for example, U.S. PAt. No. 4,390,784 of Browning, et al, issued Jun. 28, 1983.
In an alternative construction the drift tube is formed by alternating rings of metal and insulating material.
The operation of a drift tube is explained in more detail in U.S. Pat. No. 3,621,240 of Cohen, et al, issued Nov. 16, 1971.
The following U.S. patents relate to one piece cell designs and sampling techniques:
______________________________________ 4,390,784 Browning, et al 4,712,008 Vora et al 4,777,363 Eiceman et al ______________________________________
As a result of extensive research it has been determined that there are a number of key factors to be taken into consideration in connection with an ion mobility detector to ensure that ions in the cell are sufficiently focused and that space charge problems are not encountered. These key factors are:
1. a distinct trasition from the smaller diameter tube portion (reaction region) to the larger diameter tube portion (drift region), with the elbow where the two tube portions meet coated in a conducting layer to act as a second repeller plate, PA1 2. the placement of the Ni-63 radioactive ionizing source on the interior of the tube so that it is aligned with a clearly defined conducting band on a surface of the tube, PA1 3. a well defined internal repeller plate, PA1 4. care in not having any portion of the cell wall, or the cements used therein, protruding into the cell volume, and PA1 5. an adjustable gate potential to allow the formation concave field lines which properly focus the ions.
Some of these factors are known in the prior art. Examples for some of the above points are the following U.S. patents:
______________________________________ 1. Wernlund et al, #3845301, Spangler et al, #4551624 Spangler et al, #4378499, and Vora et al, #4839143 2. Wernlund et al, #3845301, and Spangler et al, #4378499 3. Wernlund et al, #3845301, and Spangler et al, #4311669 4. Ordinary knowledge in the prior art. ______________________________________
The adjustable gate potential, however, is not demonstrated in any of the prior art.